The increasing speed with which electro-optical devices can operate has created the need for correspondingly fast devices to control them. A notable example are wavelength tunable, single-mode, semiconductor lasers, which are key components in the rapidly expanding field of fiber optics, the applications of which include wavelength division multiplexing based communication systems, measurement applications, sensor applications, and sophisticated optically controlled microwave systems. An intense effort has been devoted to optimizing and manufacturing such tunable lasers, but the predictability and performance of such lasers, as well as that of the circuit drivers needed for them, have not been explored carefully. An important example of such a laser is the tunable super structure grating distributed Bragg reflector laser, whose tremendous speed in switching from one output wavelength to another offers the scope for applications which the optics community is only beginning to explore. However, to operate a super structure grating distributed Bragg reflector laser, one needs external circuitry to separately generate four control currents, the so-called front and back currents which control the effective cavity size of the laser, a phase section to control the phase of the laser""s optical field, and the laser""s gain current, which turns the laser on and controls its output power. To exploit the promise of this laser, one needs circuitry that can generate these currents with speed comparable to the laser itself, circuitry that can generate these currents accurately to ensure precise wavelength control, and circuitry that is re-programable, to permit updating circuit information as the operating parameters of the laser vary over time.
Accordingly, it is an object of the invention to provide a circuit which can output highly accurate and precise signals.
Another object is to provide a circuit which is very fast, and thus can controllably switch between preselected outputs rapidly.
Another object is to provide such a circuit whose output parameters are continuously updatable.
Another object is to provide a laser system whose output wavelength is controlled by such a fast, accurate, and reprogrammable control circuit.
In accordance with these and other objects made apparent hereinafter, the invention concerns a signal generator, having a data input, and a buffer which has a digital storage device which can store a plurality of digital words in a corresponding plurality of storage addresses. The data input and the buffer are co-operatively disposed to permit the data input to selectably address one or more of the plurality of storage addresses so as to cause the buffer to output one or more selected digital words.
Because the circuit is digital, its output, unlike that of an analog circuit, has fixed values, and hence is inherently more immune to noise. Furthermore, its accuracy is an arbitrary choice determined by the bit resolution one selects. Being digital, the outputs of the circuit are updatable simply by reading new values into the digital storage device, e.g. new settings corresponding to the drive currents for a laser whose operating parameters have drifted over time. Finally, being digital, the response of such a circuit can be extremely fast, and, moreover, computer controllable.
These and other objects are further understood from the following detailed description of particular embodiments of the invention. It is understood, however, that the invention is capable of extended application beyond the precise details of these embodiments. Changes and modifications can be made to the embodiments that do not affect the spirit of the invention, nor exceed its scope, as expressed in the appended claims. The embodiments are described with particular reference to the accompanying drawing, wherein: